Various processes utilize reactive gases carefully introduced into a processing chamber. For example, in the semi-conductor industry, precisely controlled amounts of a processing gas are often carefully introduced into a processing chamber for reaction with certain material(s) on a wafer to produce desired structures or devices. Some of these gases would be liquids at the process temperature and pressure and are generally classified as low vapor pressure liquids. It is not uncommon for a process to be run at a temperature and pressure where one of the required materials will not exist in purely gaseous form. The typical method of providing such materials in vapor form uses a device referred to as a bubbler. A bubbler is a heated container partially filled with the liquid in question. A gas is bubbled up through the liquid and combines with the vapor from the liquid. The resulting gas/vapor mixture is then introduced into the process. Bubblers are not ideally suited to the precise control required for introduction of vapor to a processing chamber because of cumbersome plumbing and heating problems associated with those devices.
Accordingly, attempts have been made to create a flash vaporizer, particularly adapted for semi-conductor processes, for providing various materials in vapor form. Flash vaporizers are disclosed, for example, in U.S. Pat. Nos. 5,361,800 and 5,371,828 to Ewing, currently assigned to the assignee of the present invention. The disclosed vaporizers include a heater assembly in thermal contact with a stack of thermally conductive, thin, flat disks biased together with a spring-loaded anvil. The heater assembly heats the disks to a temperature in excess of the flash point of the liquid to be vaporized at the process pressure. Liquid is supplied from a pumping system through a tube passing through the center of the coaxially stacked disks and is forced between the parallel disks, against the bias of the spring-loaded anvil. The liquid is heated by the hot surfaces of the disks to a temperature above its flash point and is vaporized.
However, there are many liquids that cannot be vaporized at the process temperature and pressure. The present invention is useful in addressing this limitation of the prior art devices.
As used herein, the term "evaporation" means the conversion of a liquid to a vapor by the addition of latent heat. "Vaporization", or "volatilization", means the conversion of a liquid to a vapor by the application of heat and/or by reducing the pressure on the material.
It is known that an equilibrium condition exists above the surface of a liquid, such that the number of molecules escaping the liquid surface equals the number of molecules re-entering the liquid. Each liquid has a characteristic vapor pressure-temperature profile. One can accelerate the vaporization process by increasing the rate at which vapor molecules escape from the liquid surface, by decreasing the rate at which liquid molecules reenter the liquid surface, and by increasing the vaporization surface area. By increasing the temperature of the liquid, the rate at which vapor molecules escape the liquid surface can be increased. By removing the escaping vapor molecules as they leave the liquid surface (thereby effectively lowering the pressure on the liquid surface), the reentry rate can be decreased. If the pressure on the liquid surface is constant, one can effectively reduce the partial pressure of the vapor molecules above the liquid surface, and thus accelerate the removal of vapor molecules from the liquid surface, by flowing a gas across the liquid surface. See, for example, U.S. Pat. No. 5,204,314 to Kirlin et al., in which a carrier gas is flowed past a heated foraminous matric element upon which the source reagent is deposited in liquid form to yield a carrier gas mixture containing the source reagent which is subsequently introduced into a processing chamber.
This vaporization process can be further optimized by increasing the vaporization surface area. For rapid vaporization, the liquid should form an extremely thin film on the vaporization surfaces, such as the heated disks in the Ewing patents. Thus, a large vaporization area and a means for accelerating the vaporization would be a desirable and beneficial combination.